1. Field of the Invention
This invention is concerned with a lattice interference-type displacement detection equipment and a displacement detection method, and particularly concerned with the lattice interference-type displacement detection equipment which makes a beam of light from a light source branch into two light waves, makes them incident at the identical diffraction point on a diffraction lattice of a scale and detects mixing waves of many light beams which are generated at the diffraction point as an electrical signal. Especially, it is concerned with an apparatus which prevents the return light from interfering with the light source.
2. Description of the Related Art
An example of a conventional high-resolution photoelectric-type encoder is a lattice interference-type displacement detection equipment which uses the technique of holography for a scale, which scale is formed of minute pitches (about 1 micrometer generally), makes use of the scale as a diffraction lattice and detects relative displacement with a high degree of accuracy. The conventional apparatus makes a beam of light from the light source branch into two light waves, makes them incident at one or two diffraction points on the diffraction lattice of the scale, and detects mixing waves of many light beams generated at the diffraction point as an electrical signal. The apparatus is classified into one which uses a reflection-type diffraction lattice and one which uses a penetration-type diffraction lattice.
The lattice interference-type displacement detection equipment which utilizes the reflection-type diffraction lattice is shown in FIG. 4. It is composed of the reflection-type scale 1A which is provided so that displacement to the direction of left-and-right in the figure is possible and has a reflection-type diffraction lattice 2A along the direction of the displacement, a laser light source 11, half-mirror 24 which makes the laser beam from the laser light source 11 branch into two divergence light beams A, B, a pair of mirrors 23A, 23B which reflect each of divergence light beams A, B and makes them incident at identical diffraction point P on the diffraction lattice 2A of scale 1A from a symmetrical direction respectively, mirror 32 reflects first diffraction light beams A1, B1 above the diffraction point P, and detector 41 converts the mixing wave from the light beam mixing means, which mixed the light beams reflected through scale 1A, mirrors 23A, 23B and half-mirror 24, to an electrical signal. Light beam divergence means 21 is composed of the above-mentioned half-mirror 24 and the pair of mirrors 23A, 23B. Light beam mixing means 31 is composed of the above-mentioned half-mirror 24, the pair of mirrors 23A, 23B and mirror 32 respectively.
Therefore, the laser beam from the laser light source 11 is divided into two divergence light beams by the half-mirror 24. After each of the divergence beams A, B are reflected at each of the mirrors 23A, 23B, they are made incident at identical diffraction point P on the diffraction lattice 2A of scale 1A from a symmetrical direction respectively. Then, first diffraction light beams A1, B1 of divergence beams A, B respectively, are generated on the diffraction point P. After each first diffraction light A1, B1 are reflected by mirror 32, scale 1A, and mirrors 23A, 23B in order, they are mixed by the half-mirror 24 and are led to the detector 41. After detector 41 makes the direction of polarization of the mixing wave which was mixed by the half-mirror 24 coincide with a polarization plate and makes it interfere, the light receiving element changes it to an electrical signal. Accordingly, total sine wave signals .phi.A of two cycles are attained from detector 41 when scale 1A displaces one pitch of diffraction lattice 2A.
The lattice interference-type displacement detection equipment which utilizes the penetration-type diffraction lattice is shown in FIG. 5. It is composed of a penetration-type scale 1B which is provided so that displacement to the direction of left-and-right in the figure is possible and has a penetration-type diffraction lattice 2B along the direction of the displacement, laser light source 11, polarization beam splitter 22 which makes the laser beam from laser light source 11 branch into two divergence light beams A, B along the direction of slant, a pair of mirrors 23A, 23B which reflect each of the divergence light beams A, B and makes them incident at identical diffraction point P on the diffraction lattice 2B of scale 1B from a symmetrical direction respectively, mirror 32 reflects first diffraction light beams A1, B1 which are generated at the diffraction point P, beam splitter 34 mixes the reflected light beams, and detector 41A, 41B converts the mixing waves to electrical signals. Here, light beam divergence means 21 is composed of the above-mentioned polarization beam splitter 22 and mirrors 23A, 23B. Light beam mixing means 31 is composed of the mirror 32 and the beam splitter 34.
Therefore, the laser beam from the laser light source 11 is divided into two divergence beams according to the direction of the slant of polarization beam splitter 22. After each of divergence beams A, B are reflected at each of the mirrors 23A, 23B, they are made incident at identical diffraction point P on the diffraction lattice 2B of scale 1B from a symmetrical direction respectively. Then, first diffraction light beams A1, B1 of divergence beams A, B respectively, are generated at the diffraction point P. Each first diffraction light beam A1, B1 is first reflected by the mirror 32 then mixed by the beam splitter 34 before being led to the detectors 41A, 41B. After the detector 41A makes the direction of polarization of a first mixing wave which was mixed by beam splitter 34 coincide with a polarization plate and makes it interfere, the light receiving element changes it to an electrical signal. The detector 41B retards the phase of a second mixing wave which was mixed with the beam splitter 34, by 90-degrees with respect to the first mixing wave which is made incident to the detector 41A with a 1/4 wavelength plate. After making the direction of the polarization of the second mixing wave coincide with the polarization plate and making it interfere, the light receiving element changes it to an electrical signal. By this, total sine wave signals .phi.A, .phi.B of two cycles which have a 90-degree phase difference are attained from detectors 41A, 41B when scale 1B displaces one pitch of the diffraction lattice 2B.
However, both of the above lattice interference-type displacement detection devices have an optical system which makes divergence beams A, B incident from the slant to scale 1A, 1B and makes the beams reflect and diffract in the direction of a right angle which problematically returns light to laser light source 11. To remove this, a polarization device and a wavelength plate and so on are necessary and moreover, because removing the return light is difficult even if these devices were so equipped, there is a problem in that output from a laser light source becomes unstable.
Also, because the beams A, B are incident from the slant to scale 1A, 1B and the first diffraction light A1, B1 reflects in the direction of a right angle, the arrangement of an optical device is thus limited. Especially, in the case of the penetration-type diffraction lattice 2B, because it is necessary to arrange mirror 32 opposite the optical components such as laser light source 11, light beam divergence means 21, light beam mixing means 31, and detectors 41A, 41B, with respect to the scale 1B, therefore there is a problem in that building the equipment is difficult.
An object of this invention is to resolve such conventional faults so as to prevent return light from interfering with the light source and to provide an optical arrangement which is different from the prior art and to provide lattice interference-type displacement detection equipment which is easy to build.